9/21/2009 @ 7:21PM

In Depth: America's Cleanest Commutes

To determine our list, we used data released this week by the Census Bureau as part of its annual American Community Survey. Last year, only 5.3% of America’s 140 million workers used mass transit to get to work. So in formulating our list, we considered all areas with at least 275,000 inhabitants and a mass transit rate better than the national average. Only 15 metropolitan statistical areas–geographic entities defined by the U.S. Office of Management and Budget for use by federal agencies in collecting, tabulating and publishing federal statistics–met those criteria. We ranked the 15 qualifying metros in three categories: percentage of workers commuting via mass transit, percentage of workers carpooling and percentage of workers driving alone. We assigned points based on these rankings to form our list.